1. Field of the Invention
This invention relates to homogeneous monomer-containing vulcanizable compositions derived from rubber latices, to processes for their preparation, and to covers of the vulcanized compositions for rolls of the types employed in the papermaking and textile industries.
2. Description of the Prior Art
Rolls having a metallic core with a relatively hard elastomeric covering have long been used in machinery for the making, handling and processing of lengths of paper and textiles. Where high roll pressures are employed, deformation of the elastomeric covering generates heat which may elevate the temperature within the cover to temperatures as high as about 200.degree. F. Service under such conditions of pressure and temperature will eventually lead to fatigue problems often seen as separation of the cover from the core and as cracks at the surface of the elastomer cover. Fillers and fiber reinforcement represent the conventional approaches to controlling fatigue in the roll cover.
In addition to resistance to fatigue such roll covers should have high abrasion resistance. Abrasion is a problem, for example, in the press section of papermaking machinery where rubber covered rolls press the wet paper web, which web typically contains abrasives inadvertently introduced with the pulp as well as abrasives intentionally added to the paper stock as fillers. Copolymers of butadiene and acrylonitrile are noted for their hardness and have been used in many applications where high resistance to abrasion is required.
The abrasion resistance of rubbers generally increases with increasing molecular weight. However, viscosity of the unvulcanized rubber also increases with increase in molecular weight. Accordingly, in conventional practice a plasticizer ("extending oil") is added to the unvulcanized rubber to lower its viscosity and to increase its workability to a point suitable for extrusion or other processing. In the case of acrylonitrile/butadiene (nitrile) rubbers, however, the addition of plasticizers has proven difficult in practice.
U.S. Pat. No. 4,143,092 and Encyclopedia of Polymer Science and Technology (Vol. 2, p. 705) note that only a limited number of "extending oils" or "plasticizers" are compatible with acrylonitrile/butadiene rubber, the majority of conventional plasticizers tending to ooze from the surface of the rubber as if from a fine-pored sponge. The patent further teaches that ester plasticizers, such as dioctyl phthalate, dibutyl phthalate dioctyl adipate, etc., while compatible with the rubber, are susceptible to leaching by ink vehicles and roll cleaning compounds. Similar leaching of the conventional ester plasticizers has now been noted to occur in applications in the papermaking and textile industries. For example, in the conventional papermaking processing bleaching agents and defoamers are commonly added to the paper stock. These agents as well as coating agents, waterproofing agents, etc., used to treat the formed paper are all potential leachants. Additionally, textile and papermaking rolls are conventionally cleaned with aromatic compounds such as kerosene which are also solvents for many conventional plasticizers.
Kent et al in U.S. Pat. No. 3,528,936 and Cowperthwaite et al in U.S. Pat. No. 3,751,378 recognize that high molecular weight polymers of butadiene, etc., may be plasticized by addition of certain polyester monomers. Both patents teach admixture of the monomer and polymer together with an inorganic filler and other ingredients on an open mill or in an internal mixer, i.e., "dry" blending with a filler. In these prior art dry blending processes, a homogeneous admixture of monomer in polymer is impossible without the inorganic filler which serves as a carrier for the monomer and as a dispersant.